Drum mixing apparatus is widely known in the art for use in preparing asphalt aggregate paving compositions. A typical drum mixing apparatus includes a drying zone wherein virgin aggregate is dried by agitating the aggregate in a flow of heated air; and a mixing zone wherein the aggregate material and any reclaimed roadway materials being recycled are mixed with liquid asphalt to form the desired mixture.
Drum mixers are generally recognized as having certain advantages in comparison to other types of asphalt plants, including continuous flow operation and relative portability for transportation between job locations. However, drum mixers produce unwanted by-products in the form of liquid asphalt vapors and airborne aggregate dust, which, as will be further explained, lead to the problems of oil accumulations in baghouses, baghouse fires, and opacity problems that result in failure to meet air pollution codes.
During the conventional asphalt-mixing process, the agitation of aggregate in a mixing drum during the drying process produces a high level of airborne aggregate dust. To control this dust, liquid asphalt has been introduced into the drum mixer to coat the dust and prevent it from becoming airborne. However, the introduction of liquid asphalt into the heated mixing drum produces liquid asphalt vapors comprising light end hydrocarbons which are stripped from liquid asphalt upon exposure to the steam and high temperatures present in the drum mixer. In the course of drying aggregate, a considerable amount of water is evaporated, and the hot exhaust gases in the drum mixer contain from 10% to as much as 35% steam or water vapor. The "light ends" which are stripped from the liquid asphalt upon contact with this steam appear as an oil buildup on the filter elements and walls of the baghouse and are also released through the stack, creating air pollution problems. Many light ends which remain as vapor through the baghouse condense after being exposed to low temperature air on discharge from the plant, and in extreme cases can result in oil stains forming on objects in areas around the asphalt plant. When the plant is operated with this type of process for a sufficient time, there is a high probability of fire occurring in the baghouse, because a spark from burning materials in the drum can ignite oil-soaked bags and damage or detroy the entire baghouse. In addition, the oil which forms in the baghouse can combine with dust to clog the filter elements so that air can no longer pass through, reducing plant productivity and creating difficult cleaning problems.
The degree of severity of the light end hydrocarbon problem varies with the amount of light ends in the asphalt and the amount of steam or water vapor present in the gas stream. The problem is even more severe when recycling reclaimed roadway materials, since it is necessary to use a softer virgin asphalt to compensate for the hardness in the recycle material. Softer asphalts by nature contain more light ends, thus increasing the severity of the already existing problem.
Drum mixers equipped with venturi scrubbers known to the art reduce the dust loading problems but do not correct the problem of light ends stripped from the asphalt. Conversely, the light end hydrocarbon problem can be alleviated somewhat by reducing the exposure time of the asphalt in the drum, but this reduced exposure increases the amount of airborne aggregate dust present in the exhaust. U.S. Pat. No. 4,103,350 made significant progress in the control of aggregate dust and light end emissions by providing a system which enabled a baghouse to be used to treat exhaust from a drum mixer. Formerly, drum mixers had customarily introduced liquid asphalt into the mixer as closely as possible to the aggregate inlet so that the aggregate became coated as early as possible to reduce dust emissions. However, introducing the liquid asphalt at this early stage maximized the stripping of light end hydrocarbons and caused a great deal of hydrocarbon smoke. By introducing the liquid asphalt into the drum mixer at a location farther away from the aggregate inlet to reduce smoking, a greater amount of aggregate dust became airborne. The U.S. Pat. No. 4,103,350 utilized a baghouse to treat the exhaust from the drum mixer, and the greater amount of airborne dust formed a "cake" upon the filter bags which helped to prevent the filter bags from becoming oil-soaked and further served to filter the light ends from the exhaust. However, in extreme cases, the oil would still collect heavily on the dust bags, creating the aforementioned filter-clogging problems; some light end emissions would still get through the dust "cake" on the filter bags, causing opacity problems; and there was still a potential problem with baghouse fires.
Drum dryers have been used in conjunction with pugmills in the prior art, so that liquid asphalt could be mixed with aggregate away from the heat and steam of the drum dryer. Asphalt plants described in U.S. Pat. Nos. 2,305,938 and 3,809,373 placed pugmills at separate locations away from a drum dryer. However, in those cases, it was necessary to mechanically convey the dried aggregate from the drum dryer to the remote pugmill. Efforts have also been made in the art to collect aggregate dust coming off a drum dryer by means of a "knock-out" box wherein the velocity of the air flow from the dryer decreases as it expands into the "knock-out" box and the heavier dust particles settle into the bottom of the box. Such an arrangement was contemplated by U.S. Pat. No. 4,298,287, but in that previous effort the "knock-out" box was disposed in a manner that made it necessary to mechanically convey the recovered dust back to the mixer for mixing with the liquid asphalt.